This invention relates to soldering techniques. More particularly, this invention relates to flux-free soldering techniques useful in micro electronic fabrication methods.
Conventional methods for adhering various elements onto heat sinks or other base assemblies in electronic fabrication processing have utilized flux solder methods and epoxy type cements. These conventional methods are satisfactory for a wide variety of usages. However, there are a number of micro electronic applications in which the organic content of the solder flux and of typical epoxy cements will create problems for the micro electronic device. One such usage involves gallium arsenide laser diodes as utilized in low pressure environments such as space satellites. These gallium arsenide laser diodes are utilized for communication systems, often in a continuous mode. In this type of application, the gallium arsenide laser diode produces a large amount of heat relative to its size which must be extracted from the laser diode chip or chip breakdown will occur. To this end, the gallium arsenide laser diodes are always heat sinked to extract this heat. Unfortunately, the combination of the heat with the low pressure environment will cause any organic matter to out-gas from the solder joint or the epoxy cement bond. This out-gassing will then redeposit upon the facets of the gallium arsenide laser diode chip, altering the reflectivity of these surfaces and degrading the performance of the device. Therefore, it has remained necessary to develop methods for adhering these laser diode chips onto the heat sinks without using an organic based compound.
One flux soldering technique has been to deposit either an indium (In) or a gold-tin (Au/Sn) alloy solder onto the heat sink using a thermal evaporation system. A laser diode chip is then positioned on top of the deposited solder layer. The chip and the surface of the heat sink are then heated by a gas jet to a temperature of about 240.degree. Centigrade for indium solder in the open atmosphere. The gas jet acts to liquify the indium solder surrounding the periphery of the laser diode chip but does not liquify the indium solder layer directly beneath the laser diode chip. This technique is sufficient to produce an adequate solder bond; however, there is a residual contamination by oxidation of the indium at the interface between the top of the indium solder layer and the bottom of the laser diode chip. The presence of this oxidation at this interface degrades the performance of the heat sink and thereby the performance of the laser diode chip itself.